The GSM Packet Radio Service (GPRS) standard was developed to provide packet data services to mobile stations. The GPRS standard enables multiple mobile stations to share the same time slot or time slots for uplink communications. When establishing a packet data session, the mobile station is assigned one or more time slots in the uplink and downlink. In the channel assignment, the mobile station is given a Temporal bit Flow Identity (TFI) and Uplink State Flag (USF).
For downlink communications, the data blocks transmitted on the downlink include a TFI in the header to identify the mobile station for which the data block is intended. The mobile stations monitor their assigned time slots on the downlink for downlink transmission and decode the data blocks that include their own TFI.
For uplink communications, a scheduler at the base station schedules the mobile stations sharing the same time slot or time slots. The scheduler indicates when the mobile station is scheduled to transmit in a given uplink time slot by including that mobile station's USF in a data block transmitted in a corresponding downlink time slot. A mobile station is allowed to transmit on the uplink when it detects its USF in the data block transmitted in the corresponding downlink time slot.
In order to reduce interference and save battery power, the mobile station may operate in a Discontinuous Transmission mode (DTX). In DTX mode, the mobile station may turn its transmitter off during periods when it does not have any data to send. For example, in voice over IP (VoIP) the user may be listening to a remote user. Because the user is not speaking, there is no data to send. Therefore, interference may be reduced and power saved by DTX mode. When the user begins speaking the mobile station may switch back to a continuous transmission mode (CTX).
Typically, when the mobile station is in discontinuous transmission mode, the scheduler at the base station is notified or may otherwise determine when the mobile station transitions into DTX mode to avoid wasting uplink bandwidth since the mobile station will not have any data to send to the base station. Similarly, when the mobile station transitions from DTX mode back to CTX mode, the scheduler needs to be notified or otherwise determine that a transition to CTX mode has occurred so that it may resume normal USF scheduling for that mobile station, which is required when the user is actively speaking. Some applications, such as VoIP, are highly sensitive to latency. Therefore, when a mobile station transitions from DTX mode to CTX mode, normal USF scheduling needs to resume as quickly as possible in order to avoid latency in the delivery of speech payload to the far end user. Excessive latency will cause speech payload to be discarded and therefore noticeable interruptions in speech (i.e. missing speech) and degradation of the perceived quality of the connection will result.